Porous anode



ited rates POROUS ANQDE Bertram C. Raynes and Merle E. Sibert, Euclid,and John T. Burwell, Jr., Gates Mills, Ohio, assignors to HorizonsTitanium Corporation, Princeton, N. 1., a corporation of New Jersey NoDrawing. Application December 14, 1953, Serial No. 398,191

4 Claims. (Cl. 204-64) This invention relates to the electrolyticdeposition of certain transition metals and, more particulariy, to theelectrolytic deposition of titanium, zirconium, hafnium, vanadium,tantalum and niobium.

In the copending application of two of us, Merle E. Sibert and John T.Burwell, Serial No. 35 8,194, filed May 28, 1953, there is described andclaimed the electrolytic deposition of the aforementioned transitionmetals by electrolyzing, as the anode of a cell, a substantially 100%density carbide of the transition metal in a fused halide bath under acell voltage below that at which any bath component, including a halideof the transition metal, decomposes with evolution of a significantamount of free halogen. In the course of this electrolysis of thetransition metal carbide, the metal component of the carbide isanodically dissolved in the bath and is transported through the bath fordeposition at the cathode under conditions which are virtually the sameas those prevailing at conventional aqueous electroplating operations.As the carbide anode becomes depleted in its metal component, and thisdepletion begins at the surface of the anode structure and developsinwardly, there remains behind a coherent carbonaceous structuresubstantially free of the metal. However, because of the dense andnon-porous structure of the anode material used in the practice ofinvention of the aforementioned application, the coherent carbonaceousstructure developed at the surface of the anode as a result ofdecomposition of the carbide has a correspondingly dense and non-porousstructure and therefore tends to isolate the inner residual mass ofmetal carbide in the anode from the fused salt electrolyte. As a result,it has been found that only about 20% of the metal constituent of such adense anode material can be anodically dissolved and deposited at thecathode before the resistance of the anode to further anodic dissolutionbecomes so high as to require a cell voltage which causes the unwantedand undesirable decomposition of some of the bath components.Consequently, after a minor amount of the metal carbide of the anodestructure has been consumed by electrolysis, the aforementioned densecarbide anodes have had to be disintegrated and reprocessed into freshanode structures.

We have now found that it is possible to produce the aforementionedtransition metal carbide anodes having onlyfabout one-half the densityof the aforementioned carbide anode structures, and we have furtherfound that these relatively low density carbide anodes are amenable toover 90% utilization in continuous electrolytic operation at cellvoltages below that at which a significant amount of any fused salt bathcomponent is electrolytically decomposed. Accordingly, our presentinvention resides in the use of a transition metal carbide anode havingan apparent density of approximately 50% of the carbide itself in anelectrolytic operation such as that described in the aforementionedSibert and Burwell application Serial No. 358,194 and in the Sibert andBurwell application Serial No. 383,401, filed September 30, 1953, inboth of which the anode is electrolyzed in afused halide bath under acell voltage below that at which any bath component decomposes with theevolution of a significant amount of free halogen.

We have found that the aforementioned transition metal carbide anodeshaving a relatively low apparent density may be obtained bysubstantially the same procedure as that described in the aforementionedapplication with the exception of particular control over the nature ofthe carbonaceous component of the carbide and over the sinteringconditions under which this anode structure is formed.

Our invention will be more fully understood from the followingdiscussion of the preparation of a low-density high-porosity titaniumcarbide anode, and the electrolytic decomposition of this titaniumcarbide anode in a fused salt diluent bath with the resultant depositionof metallic titanium at the cathode of the electrolytic cell. It must beunderstood, however, that this discussion is directed to titaniumcarbide merely in the interest of simplicity and that what is said herewith regard to titanium applies generally to each of the othertransition metals, zirconium, hafnium, vanadium, tantalum and niobium.

The essential chemical characteristic of the titanium carbide which maybe used as the titaniferous anode material in practicing our invention,in addition to its being substantially free of impurities other thanuncombined carbon, is that it contains no oxygen in the form ofincompletely reduced titanium oxides or in any other form. When producedin the presence of carbon monoxide or carbon dioxide, titanium carbide,which is formed by a high temperature reaction between titanium oxideand carbon, tends to retain in the residual carbide product enoughoxygen to disqualify it for use in the practice of the invention. Thus,the mere presence of a stoichiometric excess of carbon during reductionof a titanium oxide at a temperature below that at which fusion occursis not by itself suflicient to produce such oxygen-free carbide and mustbe supplemented by carrying out the reduction under vacuum conditions soas to remove from the reaction zone any carbon dioxide or carbonmonoxide, or both, as rapidly as formed. Therefore, the titanium carbidewhich is useful in practicing our invention will generally, but notnecessarily, contain a small amount, usually about 0.5 to 2% by weight,of free carbon. This free carbon, it will be understood, is a deliberateand extraneous contaminant when it is present in the titanium carbideanode material.

The physical characteristic of the titanium carbide anode materialrequired for the practice of our invention is that the small crystals oftitanium carbide forming the anode be firmly sintered into a rigid,relatively strong mass having a porous structure and an apparent densityapproximately one-half that of the titanium carbide itself. It isimportant that the adjoining crystals of titanium carbide in the anodebe firmly sintered together while at the same time the essential porousstructure of the titanium carbide mass is preserved. Thus, because ofthe porosity of the carbide anode material, relatively free movement ofelectrolyte and dissolved titanium metal is permitted throughout thecarbide mass, and, because of the firmly sintered nature of the anodematerial, more than of the titanium component of the titanium carbideanode material may be electrolytically removed therefrom With outsignificant destruction of the residual carbide-carbon structure. Theelectrolytic decomposition of such a porous titanium carbide anode underthe conditions set forth herein results in the transference of thetitanium component of the carbide anode to the cell cathode whileleaving the carbon component of the carbide as a black, solid,self-coherent mass in the physical shape of the original anode.

A titanium carbide anode having the aforementioned chemical and physicalcharacteristics can be readily produced from raw materials eitherdirectly in a single operation orless directly in an operation involvingfirst the production of titanium carbide and then the formation of ananode from this carbide material. Irrespective of the method employed toform the titanium carbide anode material, however, the raw materialsthat must be used are purified titanium dioxide (or other oxide oftitanium) having a particle size of about 20 microns or smaller andpurified carbon in the form of graphite having a particle size of 44microns or less (i. e. minus 325 mesh Tyler Standard). These purifiedand finely divided raw materials comprising a mixture of stoichiometricproportions of the titanium oxide and graphite, together with about 1%to 2% excess graphite to insure a minimum of oxygen in the finalproduct, are brought into intimate contact with one another by means ofa ball mill or other grinding device. The resulting mixture of titaniumoxide and graphite is then treated in either of the followingalternative procedures to manufacture a carbide anode material suitablefor the practice of our invention.

In the more direct procedure for the manufacture of the anode material,the mixture of titanium oxide and graphite is mixed with about 2% of abinder such as methyl cellulose and with suflicient water to form aplastic moldable mass. The plastic mixture is then extruded, pressed orotherwise formed into the rectangular slabs, cylindrical rods, coarselumps or other shapes required of the anode. The formed anodes are driedand then are heated in a controlled atmosphere furnace to convert theraw materials into the desired titanium carbide anode material. Caremust be taken to remove the carbon monoxide evolved during theconversion of titanium oxide to titanium carbide from the zone of thereaction substantially as rapidly as it is formed in order to preventthe inclusion of oxygen in the final carbide anode material. Moreover,the temperature should be closely controlled to insure firm sintering ofthe crystals of titanium carbide formed by the reaction while at thesame time avoiding such drastic sintering or fusion of these crystalswhich would destroy the essential porosity of the anode material. Theatmosphere of the furnace is evacuated until the pressure therein isreduced to about 20 microns of mercury, and the temperature of thefurnace is raised to within the range of about 2l00 to 2200 C. and ismaintained Within this range throughout the reaction period whilecontinuing active vacuum pumping. This sintering operation, it will benoted, is carried out in the complete absence of any sintering aid, suchas the alkali and alkaline earth metal halides, which are used inpracticing the method of the aforementioned Sibert and Burwellapplication, Serial No. 358,194 to produce a high density carbide withthis relatively low sintering temperature range. After the conversion ofthe raw materials to the desired porous titanium carbide anode iscomplete (as indicated by the cessation of evolution of carbon monoxidefrom the anode) the furnace is allowed to cool while maintaining thevacuum. After reaching room' temperature, the vacuum is broken and theresulting sin- -tered anode material is removed from the furnace.

Alternatively, a titanium carbide powder may be prepared from themixture of titanium oxide and graphite, and this powder may then beformed into the desired low density carbide anode material. In thisalternative pro cedure, the mixture of titanium oxide and graphite isplaced in a controlled atmosphere furnace and the air within the furnaceis evacuated by means of a high speed vacuum pump until the pressuretherein is reduced to about 20 microns. The mix is thereupon heated to atemperature within the range of about l500 to 2000 C. for about 2 hourswhile continuing active vacuum pumping in order to remove the evolvedcarbon monoxide from the zone of reaction. On completion of the reactionthe furnace is cooled, the vacuum is broken, and

the carbide material is removed. The resulting titanium carbide materialis then crushed to minus 40 mesh (Tyler Standard) and is mixed withabout 2% of a binder such as methyl cellulose and with sufficient waterto form a plastic mass. The plastic carbide mass is formed into thedesired anode shape, is dried to remove excess water, and then is firedin a controlled atmosphere furnace to sinter the carbide and form thedesired porous anode material. The firing should be conducted under avacuum of about 20 microns in order to avoid the occlusion of oxygen(from carbon oxides) in the titanium carbide product, and thetemperature at which the firing is conducted should be closelycontrolled so that the carbide mass is firmly sintered without drasticsintering or fusion in order to preserve the porosity and low density ofthe anode material. The temperature of the furnace should therefore bemaintained within the range of about 2100 to 2200 C. throughout thefiring period and in the absence of any mineralizer or other sinteringaid such as is used in forming a high density carbide pursuant to themethod described in the aforementioned Sibert and Burwell application.On completion of the sintering operation, the furnace is cooled, thevacuum is broken, and the carbide anode material is removed for use inthe electrolytic process.

As hereinbefore noted, the titanium carbide anode material producedaccording to either of the foregoing procedures is a rigid,self-supporting material having an apparent density of approximately 50%that of the titanium carbide of which it is formed. That is, theapparent density of the anode material is approximately 2.2 gm./cc. ascompared to the actual density of titanium carbide, which is about 4.25gm./cc. The low density and porous structure of the anode material isdue to the use of highly purified and finely divided raw materials, thegraphitic form of the carbon constituent of the anode, and the closecontrol of the sintering temperatures employed. An average analysis of atitanium carbide anode produced in accordance with our procedure showsless than .02% oxygen, less than .02% nitrogen, less than 005% hydrogen,and substantially theoretical amounts of titanium and carbon.

The high purity carbide anode material is electrolytically decomposed ina fused salt diluent bath composed of a mixture of alkali metal halidesand a halide of the transition metal, the bath composition and theelectrolytic condition being those described in the first mentionedSibert and Burwell application. The cell voltage, as noted hereinbefore,is maintained below about 3 volts in order to prevent the electrolyticdecomposition of any of the fused salt components of the bath. Theanodic decomposition of the carbide anode results in the dissolution inthe fused salt bath of the transition metal component of the carbideanode. This transition metal is transferred through the bath and isdeposited on the cathode in the form of firmly adhering crystals. Thecathode, which may be formed from a metal such as iron that isunaffected by the fused salt bath,'is periodically removed from the bathand a new cathode is substituted therefor. The electrolysis may becontinued without interruption, except for a the substitution of freshcathodes, until at least of the transition metal component of thecarbide anode material has been anodically dissolved in the fused saltbath and (except for mechanical losses) has been deposited at thecathode.

The fused salt baths which may be used in the practice of our inventionmay vary considerably in composition. For example, the fused salt bathmay be composed, in addition to the transition metal halide, of one or amixture of the chlorides, bromides, iodides and fluorides of alkalimetals such as sodium and potassium. The titanium (or other transitionmetal) halide may be a chloride, bromide, iodide or fluoride and may beasimple halide or a complex halide such as a double fluoride of titaniumand an alkali metal (also known as an alkali metal fluotit'anate).- Thetitanium halide should bepresent in the bath in the amount of at leastabout by weight and generally up to about 25% by weight. The presence offluorine in the bath in the form of such an alkali metal-titanium doublefluoride, or of a simple alkali metal fluoride, promotes the formationof larger particles of cathodically deposited titanium than that whichis obtained by any one or a mixture of the other halides. Except forthis special effect of an added fluoride, the specific composition ofthe bath appears to have no effect upon the quality of the titaniummetal deposited. Illustrative bath compositions which are useful in thepractice of our invention are set forth in the following table, thenumerical values under each salt heading representing the paihts byweight or percentage of each component in the K|TiF| NaOl KCl N aBr KBrNaI K1 Decomposition of a titanium carbide anode during electrolysisproceeds with the deposition of metallic titanium on the cathode and thedevelopment of a residual carbon structure or regulus at the anode. Whenthe titanium carbide anode has been formed under optimumtemperatureconditions, there is a negligible tendency for this carbide-carbonresidue to become detached from the anode and enter into the bath.However, it is possible to guard against possible contamination of thetitanium de posit with liberated particles of carbon or titanium carbideby interposing an inert mechanical barrier between the anode and thecathode as mentioned hereinbefore. Such a barrier may comprise, forexample, a graphite partition pierced with a number of very fine holeswith the top of the partition positioned below the surface of the fusedsalt bath so as to insure the presence of a low resistance electricalpath through the bath. In general, however, the provision of atrough-shaped well or the like in the lower portion of the cellstructure is sufficient to collect any particles of the carbon residuewhich become detached from the anode. Such a collector of carbonparticles may be provided in conjunction with a mechanical barrier bymounting a graphite cylinder, pierced with a number of fine holes,concentrically about the anode with the upper end of the cylinderpositioned below the surface of the fused salt bath and with the lowerend of the cylinder either embedded in the cell bottom or in a layer ofsolid bath composition maintained in the solid state by water cooling ofthe lower portion of the cell.

The electrolysis is carried out under purified argon in which alloxygen, hydrogen, water vapor, nitrogen, and the like have beeneliminated by conventional cleaning techniques well known in thetechnical arts. When titanium carbide anodes are properly made theelectrolytic reaction proceeds quietly as long as a significant amountof titanium carbide remains in the anode structure. Most eficientutilization of the titanium carbide dictates a physical shape such as toprovide a relatively large surface arca-to-volume ratio. With theseconditions properly observed, at least 90% of the titanium content ofthe titanium carbide immersed in the fused salt bath may be deposited onthe cathode without interrupting the electrolysis or raising the cellvoltage above about 3 volts.

The cathodically deposited titanium metal is formed as a tightlyadhering, densely deposited and well-crystallized metal. The cathodewith the deposited titanium metal and adhering fused salt isperiodically withdrawn into a separate cell chamber so that it may becooled out of contact with air, whereupon a fresh cathode is immediatelyExample I Titanium carbide anode material was prepared by mixmg puretitanium dioxide with an amount of finely divided graphite about 1% inexcess of that required to convert all of the titanium dioxide totitanium carbide. The particle size of the titanium dioxide in themixture was less than 20 microns and the particle size of the graphitewas less than 44 microns. The mixture of titanium dioxide and graphitewas ground together'in a ball mill and then was mixed with about 1% ofmethyl cellulose binder and with enough water to wet the mixture. Theresulting plastic mass was then formed into a rectangular anode shapeapproximately 3 x 4 x 1 inch. The anode shape was placed in a controlledatmosphere furnace connected to a high speed vacuum pump and thepressure within the furnace was reduced to below about 20 microns. Thefurnace was heated to a temperature of about 2100 C. and thistemperature was maintained for about 2 hours. Continuous active vacuumpumping removed carbon monoxide from the furnace substantially asrapidly as it was evolved from the reaction mass. At the end of thereaction period, the titanium dioxide and graphite had been converted toa sintered mass of fine titanium carbide crystals. The apparent densityof the titanium carbide anode material was 2.2 gms./cc., orapproximately 50% of the density of titanium carbide itself. Thesintered carbide anode weighed 330 grams of which 263 grams comprisedthe available titanium constituent thereof.

The sintered carbide anode was placed into a previously prepared fusedsalt diluent bath. The composition of the fused salt diluent bathcomprised 60 parts by weight of reagent grade sodium chloride and 9parts by weight of pure anhydrous potassium fiuotitanate. The fused saltbath, the temperature of which was maintained at about 850 C., hadpreviously been subjected to a purification electrolysis to removetherefrom all traces of oxygen and water. The electrolysis of thetitanium carbide anode material was carried out at the aforesaidtemperature at a cell voltage of between 2.5 and 3.0 volts. The titaniummetal anodically dissolved at the anode was deposited on a cathodeformed of a one-inch diameter steel rod. During the course of theelectrolysis the steel cathode became heavily coated with a layer ofdeposited titanium metal necessitating replacement of the cathode with afresh cathode at frequent intervals. The cathodic deposit of titaniummetal on each, of the cathodes was removed therefrom under conditionsthat prevented the contamination of the metallic deposit withatmospheric oxygen.

Of the total of 263 grams of titanium available in the titanium carbideanode, 248 grams of crystallized titanium metal was deposited at thecathode. This amount of titanium deposited at the cathode represented a94% recovery of the titanium content of the carbide anode. The finalweight of the carbide anode was 78 grams, of which 66 grams wasgraphitic carbon. The average overall current efficiency wasapproximately 60%. The metallic titanium recovered from the anode waswashed to remove adhering salts and the resulting metal crystals weremelted under an inert atmosphere to form an ingot weighing 218 grams.The loss of 30 grams in weight of metal deposited at the cathode was dueto metal fines washed away and other mechanical losses. The titaniummetal had a Brinell hardness of -180.

Example 11 p The operation described in- Example I was repeated foreach; of tho-other transition elements, zirconium, hafnium, vanadium,tantalum and niobium, the onlyvariations being that the carbide ofthetransition element was substituted; for titanium carbide and that thepotassium double fluoride of the respective transition element-Was;substituted for the potassium titanium fluoride referred to-inExample 1. Electrochemical efiiciencies and metaI recovery: efficiencieswere substantiallyathesame for each of these transition elements as theywere. in the case of titanium.

We claim:

1-. In a method of electrolytically depositing a metalof the groupconsisting of titanium, zirconium, hafnium, vanadium, tantalum andniobium, which comprises the steps of preparing an anhydrous fused saltbath consisti ng essentially ofat least one alkali-metal halide and atleast about byweight of ahalideof said metal, introducingintosaid bath acarbide of said metal robe-electrodeposited, passing an. electrolyzingcurrent through the fused bath betweensaid'carbide and a-cathode inelectrical contact with said bath, and recovering the resultantcathodically deposited metal; the improvement which comprises: providingsaid carbide asa sintered porous rigidmass having a density aftersintering of approximately one-- half that of the solid metal carbideand having a self-coherent carbon skeleton which remains after removalof metal from the mass during said electrolysis.

2-. Ina method of electrolytically depositing titanium, which comprisesthe steps of preparing an anhydrous fused salt bathconsistihgessentially of at least one alkali metal halide and at least about 5% byweight of a halide of titanium, introducing into said bath a titaniumcarbide, passing an electrolyzing current through the fused bath betweensaid carbide anda cathode in electrical contact with said bath, andrecovering the resultant cathodically deposited titanium; theimprovement which. comprises: providing said carbide as a sinteredporous rigid mass having a density after sintering of approximatelyone-half that of the solid metal carbide and having a s elt-coherent;carbon skeleton which re ains. a er remo al- Q i anium from the m ss d rgis id lec o ysi 3.. In a method oi el ectrolytieally depositingtitanium,

which mpri h st p ofr epar ng ma hydmnsfiu d salt a consisting e e tl-yet a kastone alkali; meta halide and at least about 5% by weight; ota. halide. of titan um, introdu i g n ba h; a i a um. ca ide. passingnro y in urren h q h the us d; bath b en said ar de and a ca ode i 'el ssal c ntact; with said bath, and, recovering. theresultant-'cathodically deposited titanium; the imp ;ovementv whichcomprises; providing said carbide asa sinteredporous rigidmasshaw g; a sty after s n e n -of appr ima ely 2-2 rem cubic e er n ha naa se fo eret a bo k le ton which remains. after; removalof the titanium from themass during said electrolysis. V

4. In a method of electrolytically depositing zirconium, which comprisesthe steps of preparing an anhydrousfusedsalt bath consistingessentiallyof at. least one alkalimetal halide and at least'a-bout 5% by weight ofahalide ef'z-ie conium, introducing into said bath a carbide of saidmetal to be electrodeposited, passing anelectrolyzing current throughthe fused bath between said carbide and acathode in electrical contactwith said bath, and recovering the resultant cathodically depositedzirconium; the improvement which comprises: providing said carbide as asintered porous rigid mass having a density after sintering ofapproximately one-half that of the solid metal carbide andhaving aself-coherent carbon skeleton which remains after removal of zirconiumfrom the mass during said electrolysis.

References Cited inthe fil'eof this patent UNITED STATES PATENTS2,325,201 Young; t ,7, 7-,-.-, July 217,, 1943 FOREIGN- PATENTS 635,267Great Britain Apr. 5, 1,950 334,475 Germany Mar. 14, 1921

1. IN A METHOD OF ELECTROLYTICALLY DEPOSITING A METAL OF THE GROUPCONSISTING OF TITANIUM, ZIRCONIUM, HAFNIUM,M, VANADIUM, TANTALUM ANDNIOBIUM, WHICH COMPRISES THE STEPS OF PREPARING AN ANHYDROUS FUSSED SALTBATH CONSISTING ESSENTIALLY OF AT LEAST ONE ALKALI MATAL HALIDE AND ATLEAST ABOUT 5% BY WEIGHT OF A HALIDE OF SAID METAL, INTRODUCING INTOSAID BATH A CARBIDE OF SAID METAL TO BE ELECTRODEPOSITED, PASSING ANELECTROLYZING CURRENT THROUGH THE FUSED BATH BETWEEN SAID CARBIDE AND ACATHODE IN ELECTRICAL CONTACT WITH SAID BATH, AND RECOVERING THERESULTANT CATHODICALLY DEPOSITED METAL; THE IMPROVEMENT WHICH COMPRISES:PROVIDING SAID CARBIDE AS A SINTERED POROUS RIGID MASS HAVING A DENSITYAFTER SINTERING OF APPROXIMATELY ONEHALF THAT OF THE SOLID METAL CARBIDEAND HAVING A SELF-COHERENT CARBON SKELETON WHICH REMAINS AFTER REMOVALOF METAL FROM THE MASS DURING SAID ELECTROLYSIS.